The evaluation of liquid mixing effects on a sieve plate using unsteady and steady state tracer techniques

Abstract

A rectangular sieve plate 5 ft 8 1 2 in. long, 13 1 2 in. wide with 3 16 in. diameter holes on 3 4 -in. triangular centres has been used to evaluate liquid mixing effects in sieve-tray columns. The degree of mixing has been measured in terms of a longitudinal eddy coefficient ( D E ) by an unsteady-state tracer technique. Using the air-water system a series of liquid mixing tests have been made with liquid hold-ups varying between 3 4 –3 1 2 in. and air rates 1–4 ft/sec. With 3 4 -in. and 1-in. liquid hold-ups the D E values were considered to be related to vertical oscillations in the liquid that developed at air velocities of about 3·0 ft/sec, the indication being that the oscillations reduced the degree of liquid mixing. The D E values obtained for liquid hold-ups of 2 to 3 1 2 in. have been correlated in terms of the liquid hold-up ( h L , ft) and the sueprficial air velocity ( U ft/sec) as follows: ▪ Longitudinal eddy coefficients ( D E ) have also been measured for the same sieve plate but using liquid cross-flowing conditions by a steady-state tracer technique. Using the air-water system, values of D E varied between 0·0135 and 0·25 ft 2/sec over the wide range of operating conditions investigated. No explanation was apparent for deviations from the eddy-diffusion model which occurred when the froth velocity on the plate fell below 0·23 ft/sec. At air rates of about 7·0 ft/sec pulsations on the plate occurred corresponding to a rapid increase in the values of D E . All the D E values obtained on the plate using liquid cross-flowing conditions have been successfully correlated in terms of the liquid hold-up and the froth momentum on the plate.